1. Field of the Invention
The present invention relates to the magnetic resonance imaging (MRI) technology and, particularly, to a method and apparatus for improving the brightness uniformity in an image.
2. Description of the Prior Art
The basic principle of MRI is that the hydrogen atoms in human tissues will be directionally aligned under the effects of a fixed magnetic field. When applying radio frequency pulses are applied to the human tissues by using a radio frequency transmitting coil, these hydrogen atoms will be displaced due to the effects of the radio frequency pulses, and after the radio frequency pulses end, these hydrogen atoms will recover their original state. During the recovering process, these hydrogen atoms will generate signals, which are acquired via a radio frequency receiving coil. An image is then reconstructed using the acquired signals.
An image is composed of pixels with each of them having a certain brightness and signal-to-noise ratio. In this case, the signal-to-noise ratio is the ratio of the signal magnitude to the noise magnitude in the pixels, where the magnitude of brightness represents the magnitude of the signal. It is well known that uniformity is a very important performance criterion for an image and for a coil. The uniformity of an image can generally be measured by two methods, one of which indicates the uniformity of the signal-to-noise ratio in the image. The other indicates the uniformity of the brightness in the image. The uniformity of a coil is measured by its sensitivity. The relationship between image brightness, signal-to-noise ratio of the image and the sensitivity of the coil is as follows.
Since the image is obtained by image reconstruction from the signals acquired by the radio frequency receiving coil, the sensitivity of the radio frequency receiving coil corresponds to the signal-to-noise ratio of the image.
There is a certain correlation between the uniformity of the signal-to-noise ratio in an image and the uniformity of the brightness in the image, but between them there are also some differences; the correlation between the two uniformities is related to the technology used for carrying out image processing.
When carrying out image processing, it is possible to adopt the technology of noise normalization, and it is also possible to adopt the technology of noise non-normalization. In this case, the noise normalization technology means that, in the case of maintaining the signal-to-noise ratio at each pixel unchanged, the noise at each pixel is normalized to make the noise of each pixel equal. For example: assuming that there are only three pixels P1, P2 and P3 in an image, whose signal intensities (denoted as S1, S2, S3 respectively), noise intensities (denoted as N1, N2, N3 respectively) and signal-to-noise ratios (denoted as SNR1, SNR2, SNR3 respectively) are as follows:                P1: S1=10 N1=1 SNR1=10/1=10        P2: S2=15 N2=2 SNR2=15/2=7.5        P3: S3=40 N3=4 SNR3=40/4=10        
The noises of the three pixels mentioned above can be normalized to 2, here it is needed for the noise and signal of P1 to be increased two fold, and for the noise and signal of P3 to be reduced two fold. After having undergone noise-normalization processing as described above, the signal intensities, noise intensities and signal-to-noise ratios of the three pixels are as follows:                P1: S1=20 N1=2 SNR1=20/2=10        P2: S2=15 N2=2 SNR2=15/2=7.5        P3: S3=20 N3=2 SNR3=20/2=10        
Theoretically speaking, if the technology used during the image processing is the noise normalization for each pixel, the magnitude of brightness seen from the image reflects the magnitude of signal-to-noise ratio, therefore, if using the technology of noise normalization to carry out image processing, the uniformity of brightness is determined by the uniformity of the sensitivity of the radio frequency receiving coil.
For a radio frequency receiving coil with high uniformity of sensitivity, for example a body coil, the brightness uniformity of the image obtained by the technology of noise normalization is certainly very high; but, for the head coil with relatively poor uniformity of sensitivity or the surface coil with even poorer uniformity of sensitivity, the brightness uniformity of the image obtained by using the technology of noise normalization will be relatively poor, which will cause adverse influences on clinical diagnosis.